The present invention relates generally to the field of implantable medical devices, and more particularly to an implantable medical device (IMD) which incorporates one or more microelectromechanical (MEM) magnetic field sensor that switches between open and closed states in response to the application of a magnetic field against the patient""s skin over the IMD.
In the field of IMDs that are implanted within the body of a human patient, such as implantable cardiac pacemakers and implantable cardioverterdefibrillators (ICDs) and the like, it is often desirable that certain operational modes and parameters of the device be altered temporarily or chronically in a non-invasive (i.e. non-surgical) manner. One of the most common methods of non-invasive alteration of operational parameters employs a miniaturized reed switch contained within the IMD housing that responds to the application of a magnetic field against the patient""s skin over the IMD, e.g., by closing switch contacts, for as long as the magnetic field persists.
The typical magnetic reed switch consists of a hermetically sealed cylindrical encapsulation housing two metallic reeds. The metallic reeds are disposed within the encapsulation such that when a sufficiently strong magnetic force is applied to the IMD from outside the patient""s body, the magnetic force draws the two reeds into electrical contact with one another, thereby completing an electrical circuit. When the magnetic field is withdrawn, the reeds separate, breaking the electrical circuit. Such an arrangement is disclosed, for example, in commonly assigned U.S. Pat. Nos. 3,805,796, 3,920,005, and 4,066,086.
Such miniaturized reed switches were first incorporated into implantable pulse generators (IPGs) of pacemakers. Reed switch closure is used in the context of demand pacemaker IPGs to switch the pacing mode to a fixed rate or asynchronous mode for follow-up and trans-telephonic evaluation of the implanted pacemaker. In addition, pacing rate, pacing pulse width, and mode changes which occur upon reed switch closure are used to indicate device function and battery status. One use of this technique was to monitor impending battery depletion through observation of a change in the pacing rate from a preset or programmed pacing rate in response to a battery voltage drop, as described, for example, in U.S. Pat. No. 4,445,512. This approach could only provide a low bandpass data channel, of course, to avoid interfering with the primary function of pacing the patient""s heart when necessary.
As digital circuit technology advanced, it was recognized that control of operating modes and parameters of implanted medical devices could be realized in digital or binary circuits employing memorized control states or operating parameter values. In order to change an operating mode or parameter value, xe2x80x9cprogrammersxe2x80x9d were developed based on radio frequency (RF) downlink data communication from an external programmer transceiver to a telemetry transceiver and memory incorporated within the IMD. Through the use of such telemetry systems, it became possible to provide uplink data telemetry to transmit the contents of a register or memory within the IMD to the telemetry receiver within the programmer employing the same RF transmission capabilities. The magnetically actuated reed switch was required to be closed by an externally applied magnetic field to enable the IPG to respond to simultaneously transmitted downlink telemetry signals to ensure that the IPG telemetry receiver would not mistakenly respond to RF EMI to which the patient might be exposed. Although a variety of programming schemes have been proposed that eliminate the requirement for the externally applied magnetic field and the reed switch closure, most IMDs and RF telemetry systems continue to employ them.
Pacemaker and ICD IPGs and other IMDs are continually being made smaller and smaller while retaining and even increasing device longevity and operating functions. Reed switches are typically the only devices with moving parts in such IMD housings, theoretically making them more susceptible than the pacemaker""s electronic components to damage or mechanical failure such as might result from vibration or mechanical shock or handling during assembly of the IMD. It is difficult to miniaturize reed switches while retaining high reliability and integrity as well as sufficient sensitivity to be responsive only to predetermined magnetic field intensities and not respond to weaker magnetic fields that the patient may encounter from other electromagnetic devices or systems While a reed switch must be sensitive enough to be responsive to a particular externally applied magnetic field, it is important that the switch not be so sensitive that it is responsive to every magnetic field to which the patient may be exposed in daily activity. As a result, the manufacturing tolerances for reed switches are low, making manufacturing costs high.
A number of alternative devices have been proposed to overcome the disadvantages of a reed switch. Commonly assigned U.S. Pat. No. 4,301,804 discloses a pacemaker IPG in which a circuit produces a strobe signal which is used to turn on a current flow through a Hall effect element once each pacemaker pulse cycle for a selected period of time. The presence of an external magnetic field alters the electrical properties of the Hall effect element (typically implemented in a bipolar integrated circuit fabrication process), so that a positive voltage is provided to the pacemaker circuitry when the element is strobed. While the Hall effect element is not a mechanical device, and is in that respect preferable to a reed switch, the Hall effect element has proven to be less sensitive than a reed switch, requires expensive processing and unique, expensive packaging, and is not compatible with standard linear CMOS processing which is preferentially used in implantable medical devices.
As an alternative to using mechanical reed-switches or Hall effect elements to detect and measure magnetic fields, it has been proposed in commonly assigned U.S. Pat. Nos. 5,438,990, 5,292,342, 5,391,188, and 5,891,180, for example, to employ split-drain field-effect transistors, sometimes called MAGFETs, for this purpose. Although similar to a conventional field-effect transistor (FET), the drain of a MAGFET is split into two isolated halves. Application of a magnetic field to a MAGFET device gives rise to a differential in the currents in the two split-drain-halves, the extent of this differential being directly proportional to the strength of the applied magnetic field.
Although MAGFETs, like Hall-effect devices, have the advantage of being solid-state devices, some problems with use of Hall-effect and MAGFETs are known. Unlike classic reed switches, the MAGFETs and Hall-effect devices draw electrical power when operating in response to a strobe signal. More complex circuitry is required as set forth in the above-referenced ""770 patent to process the MAGFET signal to provide adequate sensitivity and to compensate for leakage and drift, than is required to process a simple reed switch signal. The classic reed switch, while overly large, has the advantage of being more consistent in operating characteristics and sensitivity to applied magnetic fields and requires no applied power and much less signal processing circuitry.
A need therefore exists for magnetic field responsive switch that mimics these favorable characteristics of the reed switch and can be greatly miniaturized and be readily incorporated into an IMD.
In accordance with the present invention, one or more magnetic field responsive MEM switch is substituted for the reed switch, MAGFET or Hall effect devices described above currently used or proposed for use in IMDs. In accordance with the present invention, two or more MEM switches can be provided that selectively respond to externally applied north (N) and south (S) polarity fields and/or to differing magnetic field strengths to generate separate single or multiple switch closed signals that cause the IMD to selectively respond and operate in separate prescribed operating modes.
The MEM switch comprises a moveable contact suspended over a fixed contact by a suspension member such that the MEM switch contacts are either normally open or normally closed. A ferromagnetic layer is formed on the suspension member, and the suspended contact is attracted or repelled toward or away from the fixed contact. The ferromagnetic layer, the characteristics of the suspension member, and the spacing of the switch contacts may be tailored to make the switch contacts close (or open) in response to a threshold magnetic field strength and/or polarity. A plurality of such magnetically actuated MEM switches are provided to cause the IMD to change operating mode or a parameter value and to enable or effect programming and uplink telemetry functions.
An IMD control and timing circuit establishes IMD operating modes in accordance with programmed operating mode and parameter value commands. The IMD control and timing circuit responds to a switch signal to effect a programmed operating mode or parameter value. A number of programming and interrogation functions may be accomplished selectively when a plurality of such magnetically actuated MEM switches each selectively responsive to applied external magnetic field strengths and/or polarities are incorporated into IMD circuitry
The magnetically actuated MEM switch offers a number of advantages. In one approach, semiconductor batch fabrication techniques are employed to achieve what is in effect three-dimensional micromachining of single-crystal and polycrystalline silicon and silicon dielectrics separately or on a common substrate with other IMD circuitry to form one or more magnetically actuated MEM switch. Manufacturing and assembly are simplified and cost is lowered. The resulting magnetically actuated MEM switch or switches are small, rugged, simple, and can be interfaced with simple switch processing circuitry. The magnetically actuated MEM switch and the simple switch processing circuitry coupled thereto lowers current drain in comparison to the very little current in comparison to the reed switch, MAGFET or Hall effect devices and the circuitry coupled therewith.